1. Field of the Invention
The present invention generally relates to digital imaging and, more particularly, to optical evaluation and resolution measurement of digital imaging cameras, especially digital cameras useful for applications such as machine vision, character or feature recognition, bar code reading and automated inspection systems.
2. Description of the Prior Art
The capture of images on various light sensitive media has been known and widely practiced for over one hundred fifty years. During the last fifty years, optical sensors have been developed to generate electrical signals directly from the illumination pattern incident on the sensor. Even more recently, there has been considerable interest in developing signals which can be directly operated upon by digital data processing equipment for various applications such as optical inspection in manufacturing processes and automatic control systems. Automatic control systems such as vehicle guidance often rely on detection, identification and tracking of visual targets which must be accomplished with extremely high reliability, particularly when concerns for public safety, operation of costly apparatus and the like are presented. Such reliability is often limited by optical performance of image capture apparatus.
Visual targets may be of any form which can be extracted from the remainder of an image within the required response time with the data processing capacity which is practical to provide. The image from which the target must be extracted may vary widely in content and lighting conditions. That is, the natural features (or “background clutter”) of the image may not be easily controllable and the appearance of those features in an image may vary widely with lighting conditions while the features of a visual target must be extracted therefrom at high speed with limited processing. For that reason, visual targets which have features presenting particularly high contrast of relatively simple geometric form such as concentric circles are much preferred at the present state of the art. When such features are used, the critical parameter of optical system performance for target detection, identification and tracking is image resolution.
Unfortunately, optical resolution is often particularly difficult and expensive to quantitatively evaluate, particularly when derived from sensors having discrete areas corresponding to individual pixels, such as are provided by charge coupled devices which are currently preferred for image capture in most applications. In such devices, the individual pixel areas may be extremely small but each pixel area is, of course, finite and the response to radiation (e.g. light) imaged upon each pixel area is effectively averaged over that individual area. Thus, the sensor, itself, imposes a limit on resolution and complicates measurement of resolution of the optical image capture system as a whole.
At the same time, available image resolution is critical to the design of the visual targets to be employed. That is, the contrasting features of a target must be of sufficient size to be unambiguously discriminated at distances appropriate to other constraints of the application and within the resolution of the image capture system such as reasonable distance over which control is to be exercised in a vehicle guidance system, the potential variation in distance of an object having features or characters to be read or the transverse dimensions of contrasting features to be detected and processed at a distance which may or may not be well-regulated. Therefore, accurate and readily available knowledge of resolution of an image capture system is essential to efficient design of reliable systems for each of an extremely wide variety of applications. Further, it is useful and important to be able to rapidly test individual image capture devices or cameras when the device is placed in service to ascertain that it can perform as intended.